A cryptographic primitive that operates on an input of fixed size is called a Fixed Input Length (FIL) primitive. For example, all block ciphers, which are common FIL primitives, operate on messages of fixed size. Informally, a block cipher is a type of encryption algorithm that transforms a block of unencrypted text (commonly called “plaintext”) into a block of encrypted text (commonly called “ciphertext”). The plaintext and ciphertext have the same length when transformed through a block cipher. This transformation takes place under the action of a secret key. Decryption is performed by applying a reverse transformation to the ciphertext block using the same secret key. The length of a block is called the block size, and, for many block ciphers, the block size is 64 or 128 bits.
However, in practice, one is faced with the situation of applying a cryptographic primitive on data of varying lengths. This situation calls for the development of cryptographic primitives called Variable Input Length (VIL) primitives. One particular situation is the need for an encryption algorithm which deals with messages of varying sizes but at the same time preserves the property that the length of the ciphertext equals the length of the plaintext. This situation is very common in Internet applications, where traffic consists of “packets” of varying sizes. If a block cipher is being used for encryption, then the blocks that need to be encrypted could be of varying lengths.
Wireless applications also vary packet sizes. This is due to the fact that the frames of data that are sent to each user may be different lengths from user to user because of the difference in the channel conditions of the users relative to the base station.
One problem associated with constructions made of VIL primitives is that encryption and decryption can take a relatively long time. A second problem associated with these constructions is that certain constructions have relatively inferior security.
A need therefore exists for techniques that provide constructions made of VIL primitives that are efficient and that provide relatively high security.